Center electrode for spark plug



June 29, 1965 A. CANDELISE 3,192,429

CENTER ELECTRODE FOR SPARK PLUG Filed Nov. 8. 1960 uvmvroa dlfrea ("022021 ise iiawg/zw'.

ATTORNEY United States Patent Office 3,192,429 Patented June 29, 1965 3,192,429 CENTER ELECTRODE FQR SPARK PLUG Alfred Candelise, Flint, Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed Nov. 8, 1960, Ser. No. 67,961 6 Claims. (Cl. 313-289) This invention pertains to spark plugs, and particularly to an improved aircraft spark plug of the general type shown in my Patent No. 2,927,238.

In prior spark plugs having cast silver centerwire assemblies and platinum wire electrodes, cracking of the ceramic insulator is prevalent. It is believed that cracks in the ceramic insulators are caused by differential expansion between the silver and the ceramic. This invention relates to an improved centerwire assembly for a spark plug which obviates cracking of the insulator.

Accordingly, among my objects are the provision of an improved aircraft spark plug construction; the further provision of an improved center electrode structure for a spark plug; and the still further provision of an aircraft spark plug having a centerwire assembly which is designed to distribute heat in a manner which obviates cracking of the insulator.

The aforementioned and other objects are accomplished in the present invention by embodying a centerwire assembly including a cast silver shell and an electrode spindle of nonuniform diameter. Specifically, the improved spark plug is of the type having an outer metal shell with a metal shielding barrel secured thereto, and a ceramic insulator secured coaxially within the shell and shielding barrel assembly. The ceramic insulator has a centerbore within which the improved centerwire assembly of this invention is disposed, the centerwire assembly having a lower external portion constituting a firing tip.

Further objects and advantages of the invention will be apparent from the following description, reference being bad to the accompanying drawing wherein a preferred embodiment of the present invention is clearly shown and which comprises a side view of the improved spark plug partly in section and partly in elevation.

The drawing depicts a spark plug comprising a metal shell 10 having a metal shielding barrel 12 secured to the upper end thereof. A ceramic insulator 14 is coaxially secured within the shell and shielding barrel assembly, the ceramic insulator 14 having a centerbore containing the improved centerwire assembly 16 of this invention.

As seen in the drawing, the shell 10 may have a pair of platinum ground electrodes, such as indicated by 18, having their outer ends welded to the bottom of the shell and their inner ends in spaced relation to the center electrode to form a spark gap therewith. The centerbore of the ceramic insulator 14 is formed with a lower section 20 and an upper enlarged section 22 and which define therebetween a shoulder 24. The lower centerbore section 20 is of substantially constant diameter and terminates in an internal flange 26, whereas the external surface of the insulator firing tip tapers gradually toward the bottom end thereof.

The improved centerwire assembly 16 comprises a spindle having a corrosion resistant platinum cylindrical firing tip 28 butt welded at 29 to a nickel rod 30. The cylindrical end of the platinum firing tip is snugly received in the lowerend of the insulator centerbore and is formed with a flange 32 which abuts the upper surface of the internal flange 26 of the ceramic insulator. The nickel rod 30 includes a cylindrical lower end portion 34, a tapered section of gradually increasing diameter 36, a cylindrical section 38 of diameter appreciably greater than the section 34, an upper reduced diameter section 46 and a flange 42. The entire shank portion of the spindle is surrounded by a body 44 of a good heat conducting metal, such as silver, which is preferably concentrically cast in situ, as described in Patent No. 2,400,917.

As alluded to hcreinbefore, spark plugs having cast silver centerwire sections and platinum wire electrodes are superior to any other presently known type of spark plug for aircraft engines of the piston type. However, the tendency of such plugs to develop minute insulator cracks adjacent the firing tip limits the useful life of the plugs. I had discovered that decreasing the mass of silver surrounding the spindle intermediate the ends thereof results in reducing the extent of silver expansion and increasing the temperature of the ceramic insulator thereby decreasing the stresses imposed on the ceramic insulator and obviating incipient insulator cracks. I have further dis coveredthat if the spindle is of uniform diameter throughout its length, the plug is unsatisfactory since it is desirable to embody a rather large mass of silver adjacent the firing tip to maintain the temperature of the insulator tip within nondestructive limits. Likewise it is desirable to have a rather large mass of silver adjacent the upper end of the spindle since in this area the temperatures of the ceramic insulator and of the silver are lower and by embodying a rather large mass of silver heat dissipation can be more readily effected.

A conductive glass seal 46 comprising a mixture of glass and nickel or copper powder abuts the upper surface of the cast silver body 44. In order to provide a durable electrical connection between the silver body 44 and the conductive glass seal, a spiral wire 48 is embodied, the lower end of the wire 48 preferably being embedded in the silver 44. The glass seal may be of the type disclosed in United States Patent No. 2,248,415.

From the foregoing it is apparent that the invention provides an improved high performance spark plug having a temperature distribution which obviates cracking of the ceramic insulator due to heat shock. In this manner the durability and reliability of aircraft spark plugs has been significantly improved.

It will be understood that while the invention has been described specifically with reference to a preferred embodiment thereof, changes and modifications may be made all within the full and intended scope of the claims which follow.

I claim:

1. A spark plug including a ceramic insulator having a centerbore with an upper section of relatively large diameter and a lower elongated section of reduced diameter, a center electrode spindle coaxially arranged in the lower section of said centerbore having a firing tip portion with one end thereof projecting from the lower end of said insulator and a shank portion projecting upwardly from said firing tip portion, to adjacent the top of said centerbore lower section, and a body of good heat-conducting metal surrounding the shank portion of said spindle and substantially filling the space between the spindle and the insulator, the shank portion of said spindle having upper and lower elongated cylindrical sections of relatively small diameter and an intermediate elongated cylindrical section of a diameter larger than the diameter of said upper and lower sections whereby the amount of said good heat-conducting metal surrounding the intermediate section of said shank portion is small in comparison to the amount of said metal surrounding the upper and lower sections of said shank portion so as to effect improved heat distribution from the spindle to the insulator.

2. A spark plug as set forth in claim 1 wherein said lower centerbore section has an inwardly extending flange at the lower end thereof and the firing tip portion of said spindle has a flange portion within said lower centerbore section abutting the flange of said insulator.

3. A spark plug as set forth in claim 1 wherein the firing'tip portion of said spindle is made of platinum, the shank portion of said spindle is made of nickel and is butt-welded to said firing tip portion, and wherein the body of good heat-conducting metal is silver.

4. A spark plug as set forth in claim 1 wherein the upper section of said shank portion is formed with an outwardly extending flange at the top thereof and wherein said shank-portion tapers between the lower and inter.-

mediate sections thereof.

5.'A spark plug as set forth in claim 1 wherein the body of good heat-conducting metal which surrounds said spindle is of relatively uniform external diameter and wherein the exterior surface of-the lower end of said insulator tapers toward the lower end thereof.

6. In a spark plug comprisin'g a ceramic insulator having a centerbore with an upper section of one diameter and a lower section of reduced diameter, a center electrode spindle coaxial'ly arranged in the lower sectionof lower section, and a body of good heat conducting metal surrounding theshank portion of said spindle andsubstantially filling the space between the spindle and the insulator, the improvement wherein said shank portion of said spindle has an'intermediate section of a first diameter, and upper and lower sections of smaller diameters than the first diameter whereby the amount of said heat conducting metal surrounding the upper and lower sections is large in comparison to the amount of said metal surrounding the intermediate section of said shank portion so as to etfect improved heat. distribution, from the spindle to the insulator.

7 References Cited bythe Examiner UNITED STATES PATENTS 3/60 Candelise 313-141 GEORGE'N. WESTBY, Primary Examiner.

ARTHUR GAUSS, RALPH G. NILSON, Examiners. 

1. A SPARK PLUG INCLUDING A CERAMIC INSULATOR HAVING A CENTERBORE WITH AN UPPER SECTION OF RELATIVELY LARGE DIAMETER AND A LOWER ELONGATED SECTION OF REDUCED DIAMETER, A CENTER ELECTRODE SPINDLE COAXIALLY ARRANGED IN THE LOWER SECTION OF SAID CENTERBORE HAVING A FIRING UP PORTION WITH ONE END THEREOF PROJECTING FROM THE LOWER END OF SAID INSULATOR AND A SHANK PORTION PROJECTION UPWARDLY FROM SAID FIRING TIP PORTION, TO ADJACENT THE TOP OF SAID CENTERBORE LOWER SECTION, AND A BODY OF GOOD HEAT-CONDUCTING METAL SURROUNDING THE SHANK PORTION OF SAID SPINDLE AND SUBSTANTIALLY FILLING THE SPACE BETWEEN THE SPINDLE AND THE INSULATOR, THE SHANK PORTION OF SAID SPINDLE HAVING UPPER AND LOWER ELONGATED CYLINDRICAL SECTIONS OF RELATIVELY SMALL DIAMETER AND AN INTERMEDIATE ELONGATED CYLINDRICAL SECTION OF A DIAMETER LARGER THAN THE DIAMETER OF SAID UPPER AND LOWER SECTIONS WHEREBY THE AMOUNT OF SAID GOOD HEAT-CONDUCTING METAL SURROUNDING THE INTERMEDIATE SECTION OF SAID SHANK PORTION IS SMALL IN COMPARISON TO THE AMOUNT OF SAID METAL SURROUNDING THE UPPER AND LOWER SECTIONS OF SAID SHANK PORTION SO AS TO EFFECT IMPROVED HEAT DISTRIBUTION FROM THE SPINDLE TO THE INSULATOR. 